1. Field of the Invention
This invention relates to a braking system for a vehicle, and particularly to a braking system for a vehicle which adjusts a braking force of the vehicle when a high braking force is needed, for example, when the performance of a booster has deteriorated, when the vehicle is traveling downhill or when the vehicle is heavily loaded.
2. Description of Related Art
Conventionally, a vehicle braking system has been made up of a brake pedal, a booster, a master cylinder, wheel cylinders, and conduits carrying brake fluid. When applying a braking force to the vehicle, a driver depresses the brake pedal. That is, a depressing force applied to the brake pedal is boosted by the booster and transmitted to the master cylinder. The boosted depressing force is converted into a fluid pressure of brake fluid held in the master cylinder, this fluid pressure is transmitted to the wheel cylinders, and a braking force is applied to the vehicle.
The braking force generated at this time is related to the amount by which the brake pedal is operated when the brake pedal is depressed, and a braking force corresponding to the brake pedal operating amount is obtained.
However, when the performance of the booster has deteriorated, because it cannot boost the depressing force on the brake pedal as much as when exhibiting its performance fully, a braking force corresponding to the brake pedal operating amount may not be fully obtained.
Also, the brake pedal operating amount needed to obtain the same deceleration is different when the vehicle is heavily loaded or when it is travelling downhill (hereinafter referred to as heavy load driving) from when the vehicle is not carrying any load (hereinafter referred to as light load driving).
That is, to obtain the same braking force as in light load driving when the performance of the booster has deteriorated or during heavy load driving, it is necessary to make the brake pedal operating amount greater than during light load driving.
In cases like this where the large brake pedal operating amount is required, the necessary depressing force to the brake pedal is large and becomes a burden on the driver. Therefore, for example when the driver is not a physically strong person this burden may be considerable.
Also, in heavy load driving, when the same braking force as in light load driving is expected from the same depressing force to the brake pedal as in light load driving, the expected braking force is not obtained and the braking distance is longer than anticipated.
Furthermore, for example depending on the friction coefficient of brake pads pressed against wheel discs by the wheel cylinders under the action of the brake fluid pressure, the resulting deceleration of the vehicle differs even if the depressing force is the same, i.e. the master cylinder pressure or the wheel cylinder pressure is the same. For example, if the pad friction coefficient is small, with the same master cylinder pressure, the vehicle deceleration is small.